The beings (which are later said to be the future of transcended humanity) that created the wormhole outside Saturn for humanity opened a doorway to the 5th dimension as Matthews character fell into the abyss. Because we can only perceive reality in a 2nd dimensional perspective (e.i. flat shapes) these beings altered fabric of their 5th dimensional universe and reformed it into the closest geometric that a human consciousness could analyze. Our brains literally could not comprehend such a dimension - in all likelihood just peering into it would drive you insane.
In our universe time is linear: it can be warped, slowed, stretched - but akin to a line on paper its 2 dimensional and always forward. But to a 4th (or in this case: 5th) dimensional creature peering into our universe, space and time become... 3 dimensional.
Our universe to Them would be like a flip book - every event that has ever happened, every infinite probability that has or will ever occur could be witnessed and revisited indefinitely. What Matthew saw was the closest our minds could conceive a 5th dimensional creatures perspective on our world - which was depicted as the bizarre tesseract sandwich of different times and events.
The assertion was gravity is a way to atleast communicate through different dimensions and did so via mores code.
After Matthew gave his daughter the means to solve the equation to save the human race- The beings (the transcendents of humanity) collapsed the tesseract and brought him back to his universe and... you know the rest